The present disclosure relates to semiconductor memory devices, and more particularly to a flash memory devices.
Semiconductor memory devices include Random Access Memory (RAM) and Read Only Memory (ROM). The RAM is volatile memory that loses stored data in the absence of applied power. The ROM is non-volatile memory that retains stored data in the absence of applied power. The RAM includes Dynamic RAM (DRAM), Static RAM (SRAM), etc. The ROM includes Programmable ROM (PROM), Erasable PROM (EPROM), Electrically EPROM (EEPROM), etc. Flash memory is a particular type of EEPROM and is conventionally classified into as NAND type or NOR type. NOR flash memory is commonly used to store code and generally operates faster that NAND flash memory. Accordingly, NOR flash memory is often used in sophisticated electronic devices, such as contemporary mobile phones, that require high-speed data processing.
Some forms of flash memory include single level memory cells using a program and an erase state to store binary data per each memory cell. Other forms of flash memory include multi-level memory cells capable of storing 2 or more data bits per memory cell.
Like many other memory device types, flash memory uses a sense amplifier to read stored data from selected memory cell(s). In general, a sense amplifier “senses” data stored in a selected memory cell by comparing an amount of a current flowing through the selected memory cell with a reference current. In order to generate this reference current, many flash memory devices come equipped with a so-called “reference memory cell”.
In order to read the current programmed state of a multi-level memory cell, a flash memory device usually applies a sequentially increasing word line voltage to a selected memory cell. The flash memory device applies this type of word line voltage to both the selected memory cell and the reference memory cell to sense data stored in the selected memory cell. Here, the word line voltage applied to the selected memory cell is termed “a main word line voltage”, and the word line voltage applied to the reference memory cell is alternately termed “a reference word line voltage”.
In a conventional flash memory device, the main word line voltage is increased sequentially, but the reference word line voltage is maintained at a predetermined level once it is increased. Using this comparative approach, the reading of a multi-level memory cell in a flash memory device usually requires a defined interval of time referred to as “a develop interval” during which the main word line voltage is increased. Thus, during a read operation the flash memory device senses data stored in a memory cell during an interval wherein the main word line voltage is maintained at a predetermined level following the develop interval for the main word line voltage.